Pancreatic cancer is an aggressive and difficult to treat disease, with an overall 5-year survival rate of 3-5%. The incidence of pancreatic cancer is increasing and it is projected to be the 2nd leading cause of cancer death within 5 years. Despite detailed knowledge of its molecular pathogenesis, targeted therapies have had minimal impact and immunotherapy has been ineffective. Activin receptor-like kinase 4 (ALK4) is a type I transforming growth factor-? (TGF-?) superfamily receptor that mediates signaling for several TGF-? superfamily ligands. Mutation or copy number loss of ALK4 occurs in 35% of pancreatic cancer patients, with loss of ALK4 expression associated with a poorer prognosis. In addition, ALK4 has been identified in an unbiased screen as a gene whose disruption enhances Ras mediated pancreatic tumorigenesis in vivo. We demonstrate that loss of ALK4 expression increases type I (T?RI/ALK5) and type II (T?RII) TGF-? receptor (T?R) levels, leading to increased activation of canonical TGF-? signaling, enhanced acquisition of EMT markers and phenotypes, and increased cancer invasion and metastasis in vivo. We also find that ALK4 selectively regulates the cell surface expression of receptors by promoting their glycosylation and processing/trafficking to the cell surface through effects on Golgi ribbon formation/extension, which may be regulated by the interaction of the Golgi regulator, GOLPH3, with myosin 18A. Based on these preliminary results, we hypothesize that loss of ALK4 function promotes pancreatic cancer progression and chemotherapy resistance by promoting Golgi ribbon formation/extension to increase T?R receptor glycosylation and trafficking to the cell surface, increasing T?R cell surface levels, downstream signaling and cancer biology. We further hypothesize that blocking these effects in pancreatic cancer patients with loss of ALK4 function may provide therapeutic benefit. We propose three Specific Aims. Aim 1: The mechanism by which loss of ALK4 promotes TGF-? signaling will be explored including defining effects on Golgi ribbon formation/extension. Aim 2: We will define whether loss of ALK4 expression in pancreatic cancer cells facilitates cancer initiation and/or progression, or resistance to gemcitabine in pancreatic cancer models in vivo. Aim 3: We will define whether pancreatic cancer specimens with ALK4 loss have increased TGF- ? signaling and Golgi ribbon formation/extension and whether loss of ALK4 creates unique vulnerabilities in these pancreatic cancer patients, which can be exploited for therapeutic benefit. These studies will define novel mechanisms by which ALK4 loss regulates TGF-? signaling and downstream pancreatic cancer biology and could identify ALK4 loss as a predictive biomarker for anti-TGF-? approaches in pancreatic cancer and other human cancers with mutation or loss of ALK4 expression. !